1. Field of the Invention
The present invention relates to a method for calibrating the discharge heat energy used in an optical fiber fusion splicing device which fuses and joins two optical fibers by heating due to electric discharge.
2. Description of the Related Art
In an optical fiber fusion splicing device, optical fibers ends are fused and joined by using high frequency discharge. When the optical fibers are fusion spliced using high frequency discharge, the minimum splice loss occurs at a particular discharge heat energy, as indicated in a graph shown in FIG. 1, which relates splice loss to discharge heat energy. Therefore, it is important to apply adequate discharge heat energy to minimize the splice loss.
In general, in a fusion splicing device of optical fibers, the discharge current is maintained at a given value while discharging, by a feed back control. The quantity of heat applied to the optical fibers can be controlled by adjusting a reference value of this feedback control current. The relation between the discharge current x and the discharge beat energy y can generally be represented by a relational expression y=f(x), which produces curves such as the ones shown in FIG. 2, where the discharge current is shown on x-axis and the discharge heat energy is shown in y-axis.
However, it has been observed that even though the discharge current may be maintained at a constant, the quantity of heat applied to the optical fibers changes gradually with usage of the discharge electrode. This is because the relationship between the discharge current and the discharge heat energy is affected by the changes in the fusion parameters caused by such factors as glass deposition on the discharge electrode, wear of the discharge electrodes and changes in discharge paths. Because the changes in the condition of the discharge electrodes often causes a change of the electrical resistance between the electrodes, the heat energy changes with usage of the electrodes. In other words, the relationship between the discharge heat energy and discharge current changes as illustrated by a curve y=f(x) in FIG. 2.
For this reason, although fusion splicing operation is carried out under a constant discharge current, actual heat applied to the optical fibers changes in practice, and splice loss often deviates from the initial splicing conditions aimed for minimum splice loss. That is, in the curve shown in FIG. 1, actual discharge heat energy applied optical fibers shifts from the minimum point.
To avoid such problems in producing a low-loss splice by fusion splicing, it is necessary to establish a constant discharge heat energy applied to the optical fibers. In order to maintain a constant discharge heat energy, it is necessary to calibrate the discharge heat energy by altering either the reference discharge current for feedback control or discharging duration.
This method of measuring the discharge heat energy is disclosed in a Japanese Patent Application, First Publication, Hei 5-150132, which is based on using dummy optical fibers before starting to weld the actual optical fibers to calibrate discharge heat energy by observing the state of fusion of the optical fiber ends.
The method of measuring the discharge heat energy will be explained with reference to FIGS. 5A-5C. First, the two optical fibers 10 are placed with a known gap L1, as shown in FIG. 5A. Next, as shown in FIG. 5B, discharge electrodes 21 are activated to generate a high frequency discharge to melt the ends of the optical fibers 10 while maintaining the relative positions of the optical fibers 10. The result is a fusion of the ends of the optical fibers 10 to cause them to retract to result in a gap of L2. The change of the gap (L2-L1), that is, retracting amount, is used to measure and calibrate the discharge heat energy generated during fusion splicing.
However, the extent of end retraction is affected by the degree of spreading of the discharge field. Therefore, the discharge heat energy measured according to the method described above, which is based on measuring the discharge heat energy according to the change of the ends gap of two optical fibers, does not give an accurate estimate of the discharge heat energy. For this reason, discharge heat energy data calibrated by the distance of end retraction do not coincide with the adequate discharge heat energy to achieve the minimum splice loss.
There is also a related patent that is an ECF function. To splice fibers having eccentric cores, if fusion splicing is carried out by aligning the central axes of the cores 11 (referred to as the core axes), as shown in FIG. 3A, the surface tension forces act on the end portions of the optical fiber to reduce the cladding axes offset of the opposing fibers 10. The resulting splice has a straight cladding axis, but the core axis has offset, as shown in FIG. 3B, and a higher splice loss is experienced by the core axes offset.
Therefore, there is a method of splicing, called eccentricity correct function (ECF) in which the self-aligning effects of the cladding axes caused by the surface tension forces on fused optical fiber are into account. In the ECF method, optical fibers 10 are aligned with intentional core axes offset of the optical fibers 10, as shown in FIG. 4A. The amount of the core axes offset of the optical fibers 10 caused by the self-aligning effect is calculated from the core eccentricities. Then, the optical fibers 10 are fusion spliced while maintaining this relative position of the optical fibers 10. Optical fibers 10 thus joined exhibits a cladding axes offset but the cores are straight as shown in FIG. 4B, thereby producing an optical fiber with good core alignment, and reducing the splice loss. The details of this technology are disclosed in a Japanese Patent Application, First Publication, Sho 60-195504.